1. Field of the Invention
The present invention relates to an oil pump system. In particular, the present invention is applicable to an oil pump system which exhibits an increasing hydraulic-pressure characteristic; namely: it produces an increasing hydraulic pressure as a revolving speed of a driving source, e.g., a crankshaft of an internal combustion engine of a vehicle, increases.
2. Description of Related Art
In oil pump systems, when a revolving speed of their rotor increases, a hydraulic oil is discharged out of an outlet port in an increasing outlet amount. As a result, oil pump systems produce an increasing hydraulic pressure.
Heretofore, as disclosed in Japanese Unexamined Utility Model Publication (KOKAI) No. 61-23,485, a conventional oil pump system has been known. This conventional oil pump system is provided with two gear pumps which are disposed integrally therein. In the conventional oil pump system, two gear pumps are actuated in order to securely discharge a hydraulic oil in a predetermined outlet amount in low revolving-speed region where a hydraulic pressure is likely to be low. Thus, the oil pump system securely produces a required hydraulic pressure. On the other hand, only one of the two gear pumps is actuated in high revolving-speed region because an outlet amount is increased so large that hydraulic-pressure increment can be expected. Thus, the oil pump system is inhibited from producing a hydraulic pressure more than required, and thereby its work efficiency is improved.
Further, another conventional oil pump system has been known so far. As illustrated in FIG. 10, this conventional oil pump system is provided with a relief valve 300. The oil pump system is further provided with a pump body 100 which includes an inlet port 101 and an outlet port 102, and a rotor 200 which is disposed rotatably in a pump chamber 105 of the pump body 100 and which has a plurality of teeth. The relief valve 300 is actuated in responsive to a hydraulic pressure in a hydraulic oil which is discharged out or the outlet port 102.
Similarly to the first conventional oil pump system, when the revolving speed of the rotor 200 is increased, the hydraulic oil is discharged out of the outlet port 102 in an increasing amount in this conventional oil pump system as well. When the revolving speed of the rotor 200 is increased to produce a hydraulic pressure of a reference pressure P.sub.1 or more, the hydraulic pressure of the hydraulic oil discharged out of the outlet port 102 overcomes the urging force of the relief valve 300, and thereby the relief valve 300 is actuated to open. Accordingly, the excessive hydraulic oil is discharged to the outside via a relief port of the relief valve 300.
Furthermore, the conventional oil pump system disclosed in Japanese Unexamined Utility Model Publication (KOKAI) No. 61-23,485 requires two gear pumps. Accordingly, it is disadvantageous from the downsizing point of view, and it is unfavorable in terms of on-boardability when boarding it on carriers like vehicle bodies.
Moreover, in accordance with the conventional oil pump system illustrated in FIG. 10, when a hydraulic pressure of the reference pressure P.sub.1 or more is produced, the relief valve 300 is actuated to open by the hydraulic pressure of the hydraulic oil delivered out of the outlet port 102, and the excessive hydraulic oil is discharged to the outside as described above. Consequently, the excessive hydraulic oil discharged to the outside is also subjected to a large hydraulic pressure of the reference pressure P.sub.1 or more, and accordingly the conventional oil pump system carries out to produce an excessive work. Thus, this conventional oil pump system is not favorable in terms of work efficiency.